This invention relates to polycrystalline diamond abrasive elements.
Polycrystalline diamond abrasive elements, also known as polycrystalline diamond compacts (PDC), comprise a layer of polycrystalline diamond (PCD) generally bonded to a cemented carbide substrate. Such abrasive elements are used in a wide variety of drilling, wear, cutting, drawing and other such applications. PCD abrasive elements are used, in particular, as cutting inserts or elements in drill bits.
Polycrystalline diamond is extremely hard and provides an excellent wear-resistant material. Generally, the wear resistance of the polycrystalline diamond increases with the packing density of the diamond particles and the degree of inter-particle bonding. Wear resistance will also increase with structural homogeneity and a reduction in average diamond grain size. This increase in wear resistance is desirable in order to achieve better cutter life. However, as PCD material is made more wear resistant it typically becomes more brittle or prone to fracture. PCD elements designed for improved wear performance will therefore tend to have compromised or reduced resistance to spalling.
With spalling-type wear, the cutting efficiency of the cutting inserts can rapidly be reduced and consequently the rate of penetration of the drill bit into the formation is slowed. Once chipping begins, the amount of damage to the table continually increases, as a result of the increased normal force now required to achieve the required depth of cut. Therefore, as cutter damage occurs and the rate of penetration of the drill bit decreases, the response of increasing weight on bit can quickly lead to further degradation and ultimately catastrophic failure of the chipped cutting element.
JP 59-219500 teaches that the performance of PCD tools can be improved by removing a ferrous metal binding phase in a volume extending to a depth of at least 0.2 mm from the surface of a sintered diamond body.
A PCD cutting element has recently been introduced on to the market which is said to have greatly improved cutter life, by increasing wear resistance without loss of impact strength. U.S. Pat. Nos. 6,544,308 and 6,562,462 describe the manufacture and behaviour of such cutters. The PCD cutting element is characterised inter alia, by a region adjacent the cutting surface which is substantially free of catalysing material. Catalysing materials for polycrystalline diamond are generally transition metals such as cobalt or iron.
Typically the metallic phase is removed using an acid leaching or other similar chemical technology to dissolve out the metallic phase. Removal of the metallic phase can be very difficult to control and may result in damage to the highly vulnerable interface region between the PCD layer and the underlying carbide substrate. In addition, in many cases the substrate is more vulnerable to acid attack than the PCD table itself, and acid damage to the metallic phase in this component will render the cutter useless or highly compromised in the application. Masking technologies are employed to protect the majority of the PCD table (where leaching is not required) and the carbide substrate, but these are not always successful, especially under extended periods of treatment.
U.S. Pat. Nos. 6,544,308 and 6,562,462 teach that the most optimal response to leaching of the PCD layer is achieved where leach depths exceed 200 μm. The highly dense nature of the PCD typically treated requires extreme treatment conditions and/or time periods to achieve this depth of leach. In many cases the masking technologies available do not provide sufficient protection damage on all units undergoing the treatment.
In order to provide PCD abrasive elements with greater wear resistance than those claimed in the prior art previously discussed, it has been proposed to provide a mix of diamond particles, differing in their average particle size, in the manufacture of the PCD layers. U.S. Pat. Nos. 5,505,748 and 5,468,268 describe the manufacture of such PCD layers.